Embryonic stem (ES) cells have the ability to grow indefinitely while maintaining pluripotency and the ability to differentiate into a multitude of different cell types. Because of these two qualities, human ES cell therapies have been proposed for regenerative medicine and tissue replacement after injury or disease. However, there are ethical difficulties regarding the use of human embryos for the isolation of human ES cells as well as problems with tissue rejection following transplantation of foreign ES cells in patients.